RELIABILITY
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Figure 1. Under magnification, the metal-to-metal interface in an electronic connector can be seen to be rough (1). Micromotion (2) abrades both surfaces, allowing corrosion to create debris particles that can separate the interfaces and prevent conductivity (3). (click to enlarge) |
Electronic connectors typically fail when corrosion of their interfacing metal surfaces creates insulating by-products that make signal transmission intermittent or impossible (see Figure 1). A close look with a microscope at the metal-to-metal interface in an electronic connector reveals that the interfaces are actually rough, not smooth as they appear to the eye.
The phenomenon of electrical conductivity occurs only over a small percentage of the metal area, at the few points where the metal surfaces are truly in contact. When these surfaces are abraded, the base metal is exposed to airborne corrosives. The subsequent corrosion creates debris particles, mostly metal oxides, that can act to separate the interfaces altogether and stop conductivity.
A worst-case example of the problem might be an inexpensive electronic toy with base-metal connectors that a child is using in a high-humidity, high-salt seaside environment and that is being subjected to frequent vibration. Since corrosion can begin—and actually be measured—in less than 60 seconds under test conditions that are considerably more benign than this example, the toy would very possibly experience connector failure in a short time. Thankfully, medical devices are better engineered than toys, although it is not beyond the realm of possibility that a portable medical device exposed to the same or similar environmental conditions might also experience a connector failure.
Predicting when an electronic connector will fail is tricky at best, but there are test protocols that are used to avoid failures. These test protocols may, for example, evaluate material characteristics and design parameters, analyze known mechanisms of failure, or use testing and failure analysis of electronic connectors to predict the potential for failure. Most large manufacturers of connectors use some form of test protocol.
Micromotion and Corrosion
To maximize the long-term reliability of electronic medical devices, system builders sometimes apply a lubricant to the connectors within the assembly. While most connectors may appear to be essentially immobile, motion—or, more precisely, micromotion—is one of the chief causes of connector corrosion. Micromotion is characterized as the repeated limited movement of two surfaces relative to each other. In connector testing, 50 µm, or 0.002 in., is the vibratory distance typically used to replicate the micromotion a device connector might experience in service. In a device in use, connectors can be subjected to small vibrations causing movement of 5–100 µm at the interface.
This degree of movement within the connector can be caused by vibration from a motor, fan, or similar source, or even simply by changes in temperature. Just turning the power on can generate enough vibration to cause micromotion of 5 µm at the connector interface. Unless a lubricant is applied, the connector surfaces will almost certainly be damaged.
Even if the surfaces are plated with gold, the problem remains. Micromotion can remove the gold and expose the base metal to corrosion. The action of micromotion can easily transfer the gold from one surface to the other. Also, gold plating is often so thin that it is porous. For any of these reasons, the likelihood of corrosion of the base metal is very high.
Connector Lubricant
Medical equipment usually has to exhibit not merely long-term reliability but uncompromising reliability. Failure is simply not acceptable in dialysis machines, for example, or in pulmonary monitors.
However, when a connector lubricant is used, it nearly always is applied only once—during electronic assembly of the system. Relatively few connectors in installed systems are readily accessible for easy lubrication, and few repair technicians carry connector lubricants with them. When connector failure does occur, in most cases it necessitates costly equipment disassembly for the purpose of replacing the corroded component.
Whichever type of connector lubricant is used in a particular medical system, the material must have several distinctive properties. Probably most important, it has to remain where it has been applied. The lubricant is useless if it migrates away from the connector, or, more specifically, from the two in-contact surfaces. Most general lubricants have little or no positional stability and work briefly or not at all on electronic connectors.
The substance must lubricate the surfaces against micromotion and also against larger excursions. And it must continue to provide effective lubrication over a fairly wide temperature range. Because the lubricant must be counted on to protect reliability during the life span of the equipment, which may be several years, it must have a low vapor pressure. That is, it must evaporate only very slowly. Other physical and chemical requirements for the material are that it be thermally and oxidatively stable.
The lubricant must operate well both on noble metals, including gold and palladium, and on such nonnoble, or base, metals as tin. Finally, it must be chemically inert—that is, nonreactive to the connector and the connector housings.
Polyphenyl Ethers
One group of lubricants meets all of these requirements handily: the polyphenyl ethers (PPEs). Most well-known lubricants are formulated from petroleum or petroleum derivatives. A few connector lubricants are based on silicone. But PPEs are wholly synthetic and therefore have little in common with petroleum-based lubricants.
An example of the superiority of performance offered by PPEs is that they remain stable and continue to act as effective lubricants at temperatures as high as 453°C (847°F), while petroleum-based and other synthetic lubricants decompose at around 200°C (392°F). PPEs are also extremely resistant to ionizing radiation, a property that has led to their widespread use in nuclear facilities and on earth-orbiting satellites.
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Figure 2. A five-ring polyphenyl ether (PPE) molecule. (click to enlarge) |
The extreme properties of PPEs are attributable to their molecular structure. The various compounds in this family contain from 2 to 10 benzene rings, with ether linkages (an oxygen atom connecting two carbon atoms) between the rings (see Figure 2). In the case of the five-ring PPE shown in the figure, various parts of the molecule are able to rotate around the four oxygen atoms linking the benzene rings. This easy rotation makes the whole molecule flexible, which in turn means that a PPE will remain fluid for a long time, rather than thickening or hardening, even under adverse service conditions.
Additionally, PPEs possess very high resonance energy, which is the energy that must be overcome in order to dissociate the molecule. Their high resonance energy is what makes PPEs stable at high temperatures and under high levels of ionizing radiation. PPE molecules are difficult to fabricate, but they are also difficult to disassemble.
PPEs and Connectors
The property of polyphenyl ether lubricants that is probably the most important for offering reliable protection of electronic connectors is their very high surface tension. Most fluid lubricants are visualized as a thin, flat film that coats a surface. But PPEs do not behave in that manner. The very high surface tension of these compounds means that they rest on a surface to which they have been applied as a crowded group of steep-sided microdroplets.
Although PPE droplets lubricate and protect the connector surface just like conventional fluid lubricants, they do not flow. Instead of migrating off of the connector they are intended to lubricate, they remain where they have been applied. PPEs exhibit very strong, very long-term positional stability.
The importance of positional stability can be seen by comparing PPEs with silicone-based connector lubricants. Silicone-based lubricants have a surface tension that is somewhat higher than the surface tension of petroleum-based lubricants, which suggests a performance advantage over the latter. However, that force is still low enough to allow the silicone lubricant to migrate from the point of application.
Migration has two consequences. First, it can result in less lubricant on the connector itself that can prevent corrosion and the buildup of corrosion by-products. Second, the silicone-based lubricant may migrate onto the printed circuit board. If the board is made of FR4 laminate, then that lubricant can attack its surface. These consequences of migration would be especially worrisome with respect to a system such as a portable defibrillator carried aboard an aircraft. That medical device would be subjected to extensive vibration and thermal excursions, and yet would be expected to function flawlessly for years.
PPEs have another property that is conducive to the long-term reliability of medical electronic devices: they evaporate very slowly. Their extremely low vapor pressure is exemplified by the performance of the five-ring PPE diagrammed in Figure 2. That compound will evaporate from a connector surface only after the passage of 40 to 50 years. Other lubricants with higher vapor pressures will evaporate much sooner.
The PPE lubricants will actually outlast the connector itself. And, as an additional benefit, connectors that are periodically withdrawn from and reinserted into an assembly are subjected to lessened mechanical forces during those operations because of the lubricant that remains in place.
A Novel Alternative
Not all electronic connectors require the top-level lubricating performance of PPEs to achieve high reliability. But until recently, a wide performance gap existed between low-end connector lubricants and PPEs, and an assembler who needed superior performance had only PPEs to choose from. This gap has now been filled, however, through the development and introduction of advanced phenyl ether (APE) lubricants.
The single most important performance property of connector lubricants, as mentioned, is positional stability: the lubricant cannot protect the connector if it has migrated to a different location. APE lubricants have a surface tension that is lower than that of PPEs but significantly higher than the surface tension of ordinary connector lubricants. When applied to the connectors in, for example, an x-ray system or a ventilator, an APE lubricant ensures reliable equipment function by remaining dependably in place at the connector interface.
One maker of medical equipment recently reported experiencing field failures that, as it turned out, were occurring because the gold on the connector surfaces was being peeled off by micromotion, exposing the base metal. The first attempt at solving the problem—which involved simply plating a thicker layer of gold—did not end the field failures. But when an APE lubricant began to be used, the failures stopped.
Conclusion
The focus of electronic connector lubrication is the interface between the two metal surfaces of the connector, a focus that both PPE and APE lubricants maintain very well. These surfaces appear smooth under ordinary magnification, but higher magnification reveals their rough and bumpy nature. Perhaps 5% of the contact area of one surface is actually contacting its counterpart. In many connectors, the true metal-to-metal contact area is much less than 5%. Keeping corrosion and corrosion by-products out of that critical and tenuous interfacing area improves the long-term reliability of medical electronic equipment.
